Seat backs, such as in aircraft or other vehicles, support a user's weight when seated. Typical airline seats have a cushioned, but rigid base and back, with a pivot between them. In a given vehicle with multiple seats, the seat backs are typically all standardized (e.g., the same size, shape, and/or stiffness). This can create discomfort for some users, such as those who are significantly taller or shorter than average, those that are significant heavier or lighter than average, and so on. Differing user postures and preferences on rigidity may also result in discomfort. Previous attempts at addressing discomfort for users have included incorporating air bladders to vary seat stiffness. However, these solutions are not well-suited for use in aircraft with changing altitudes because the air bladders may lose or gain stiffness during climb and descent due to changes in air pressure, and thus may not always be capable of maintaining the user's desired rigidity.